The invention relates to semiconductor laser transmitters for use in fiber-optic communication systems.
Directly modulated semiconductor lasers are attractive for use as transmitters in optical communication because they are compact, have large response to modulation, and are integrable. In addition they are inexpensive compared to externally-modulated transmitters, which require an intensity modulator, usually LiNbO3, following the laser. However, they suffer from the major drawback that their outputs are highly chirped. Chirp is the rapid change in optical frequency or phase that accompanies an intensity modulated signal. Chirped pulses become distorted after propagation through tens of km of dispersive optical fiber, increasing system power penalties to unacceptable levels. This has limited the use of directly modulated laser transmitters to propagation distances of only tens of km at 2.5 Gb/s as described by P. J. Corvini and T. L. Koch, Journal of Lightwave Technology vol. LT-5, no. 11, 1591 (1987). The distortion-less transmission distances become even shorter for higher bit rates.
An alternative transmission system that produces reduced chirp was described by R. E Epworth in UK patent GB2107147A in which the modulated laser is followed by an optical frequency discriminator. The laser bias current is modulated by the electrical signal to produce small amplitude modulation as well as modulation of the laser frequency. The discriminator then converts the FM modulation to AM modulation. Epworth cites classic optical discriminators, namely Mach-Zehnder interferometer, Michelson interferometer, and two Fabry-Perot resonators for implementation of this invention. N. Henmi describes a very similar system in U.S. Pat. No. 4,805,235, also using a free-space interferometer.
The above are all free-space discriminators comprising mirrors and partial reflectors which are bulky and required mechanical feed-back control for their stabilization. Also coupling of light from fiber to these devices and back introduces loss as well as additional optical components. Furthermore tuning the discriminator for optimum position requires a mechanical adjustment of the phase differential in one arm of the interferometer. As stated in UK patent GB 2,107147A, the modulated laser frequency varies by only 10-20 GHz, so to obtain a IO GHz frequency discrimination, a .about.1.5 cm free-space delay is needed, which requires use of mechanically driven parts. Piezoelectric elements can only make small motions and are known to drift, requiring further stabilization circuits.
N. Henmi in U.S. Pat. No. 4,805,235, also sites the use of a diffraction grating discriminator. To obtain 10-20 GHz frequency discrimination as stated above, the diffraction grating has to have a resolution of a few GHz to discriminate between the "Is" and "Os" in a digital system. From "The Feynman Lectures on Physics" vol. 1, Addision Wesley, MA (1963), a frequency resolution .DELTA.v.about.1 GHz requires a time difference between extreme paths of .DELTA.t.about.1/.DELTA.v.about.1 ns corresponding to a larger than 1 ft wide diffraction grating. As described in U.S. Pat. No. 4,805,235, light beams of different frequency components have to diffract before they are separated by slits a distance away from the diffraction grating. This makes for a bulky device. Also, as for the other free-space optics discriminators mentioned above, it suffers from fiber coupling loss and requires mechanical tuning.
In U.S. Pat. No. 5,317,384, J. P. King describes a fiber Mach-Zehnder interferometer as discriminator comprising polarization-preserving fibers, couplers, and a fiber delay line. This discriminator is an improvement over the previously mentioned discriminators in that it is in-fiber and is polarization insensitive. Discrimination is achieved by making one arm of the interferometer longer length of fiber (we calculate, by .about.1 cm longer for 10 GHz variation). This has the disadvantage that the discriminator cannot be tuned. Also, it is a complicated structure comprising two fiber polarization splitters, two fiber couplers, four cross splices and a regular splice.
Furthermore the transfer function of a Mach-Zehnder discriminators is limited to being sinusoidal. For digital applications, a sinusoidal transfer function is not optimum and leads to distortion if the frequency excursion of the laser exceeds the range of the transfer function between a first maximum and minimum.
In addition, the return-to-zero (RZ), modulation format is being considered for use in 10 Gb/s and 40 Gb/s long haul fiber optic systems. Record propagation distances exceeding 28000 km have been demonstrated at 10 Gb/s in a dispersion-managed soliton system experiments using this format. One of the obstacles in deployment of RZ systems is the lack of inexpensive, compact, high bit rate sources of nearly transform limited optical pulses with low timing jitter.
A number of techniques for pulse generation exist, but all have major drawbacks. Gain-switched and filtered distributed feed-back lasers suffer from timing jitter and extreme chirp. Mode-locked semiconductor lasers using external cavities are difficult to engineer, typically have fixed bit rate that is locked to a cavity length, and are specially expensive. Schemes using intensity modulation of CW signal suffer from bias drift of the external intensity modulator.
P. V. Mamyshev has shown that removing the central frequency components of a phase modulated CW signal produces transform limited pulses at high repetition rate. This technique is simpler than all previously mentioned sources and requires a CW laser and a phase modulator, typically LiNbO.sub.3, modulated at the desired bit rate, and a low pass or high pass optical filter. In addition, bias drift is not a problem for the phase modulator. In order to produce relatively short pulses at 10 Gb/s, the modulator must impart a phase shift of .about..pi.-1.5.pi. at this bit rate and therefore requires excessive RF power of 27-32 dBm. In addition, LiNbO.sub.3 modulators are expensive and somewhat bulky for to the limited space on a commercial transmitter card.
Further, long-haul DWDM systems require compact, low-chirp transmitters with stabilized frequency. External modulation transmitters are expensive and require additional optics for wavelength stabilization. Directly modulated external-cavit fiber-grating lasers have frequency stability, but are limited to a penalty-free propagation distance of -117 km in non-dispersion-shifted (NDS) fiber at 2.5 Gb/s.
Recently, Chang-Hee Lee et al., Technical Digest, CLEO' 95, vol 15, paper CtuI10 (incorporated by reference) used a band-pass filter to reduce the chirp of a directly modulated laser obtaining a 1.5 dB penalty after 200 km of NDS fiber at 2.5 Gb/s. P. A. Morton, et al., Electron. Lett. Vol. 33, p. 310 (1997)(incorporated by reference) demonstrated 38.5 km propagation in NDS fiber at 10 Gb/s using this technique. Filtering has also been considered the FSK demodulation. A similar method, called dispersion-supported transmission (DST), uses fiber dispersion for frequency to amplitude conversion of a directly modulated signal. In this scheme, transmitter chirp has to be adjusted to a given fiber dispersion and length, and an unconventional receiver is needed.